In FIG. 1, a conventional fuel cell stack 14 has about 300 fuel cells; only a fuel cell 16 and portions of adjacent fuel cells 18, 19 are shown. The fuel cell 16 comprises a membrane electrode assembly (MEA) 21 which includes a proton exchange membrane together with cathode and anode catalysts. An anode support plate 22 is adjacent to a porous anode water transport plate 23, which includes fuel flow field passages 26 and grooves 28 which make up coolant water passageways 29 when matched with grooves 30 on an adjacent cathode water transport plate 31. Similarly, a cathode support plate 32 is adjacent to a porous cathode water transport plate 34 which has grooves 35 which will form water passages 38 when matched with grooves 36 of an additional anode water transport plate 37 of the next fuel cell 19. In many PEM fuel cells, the water passages will be in only one water transport plate, the contiguous water transport plate of the adjacent cell being flat.
The cathode water transport plate 31 of the cell 18 has oxidant reactant gas passages 40, and the cathode water transport plate 34 has oxidant reactant gas flow field passages 41. The fuel cell 18, only partially shown, includes a membrane electrode assembly 42, an anode support plate 43, and a cathode support plate 44, the remainder of this fuel cell being broken away for simplicity.
In conventional PEM fuel cell power plants, the water management system includes the porous water transport plates 23, 31, 34, 37 which have reactant gas channels 26, 40, 41, 47 on one side and water channels 29, 38 on the other side. Referring to FIG. 2, a conventional fuel cell stack 50 may include about 300 cells similar to the cells 16, 18 and 19 in FIG. 1, and may have a water management system in which water enters the water inlet manifold 52, passes through the channels 29, 38 (FIG. 1), exits a water exit manifold 53, and traverses either through a heat exchanger 54 or a bypass valve 55. The bypass valve 55 can be selectively opened to a varying degree in response to a controller 60 which may receive temperature information over a line 61 from the stack 50. A water pump 63 will cause circulation of water through the stack and the heat exchanger. There may be an accumulator 62 in some systems.
This apparatus takes up space which is scarce, particularly in electric vehicles powered by a fuel cell. Furthermore, the parasitic power requirement of the electric pump detracts from the overall efficiency of the fuel cell process.
The conventional wisdom related to starting fuel cells, particularly for automotive applications, when they have been subjected to freezing temperatures, is to heat the fuel cell prior to starting it, which means a significant delay before the vehicle can be operated. A concomitant goal has been to reduce the amount of water which remains in the power plant, thus to reduce the amount of water which has to be thawed and heated prior to operating the fuel cell power plant. In commonly owned, copending U.S. patent application Ser. No. 10/465,006 filed Jun. 19, 2003, a PEM fuel cell stack has reactant gas flow fields and water flow fields in porous water transport plates, and separate coolant flow fields, the coolant being an antifreeze solution. Water management is accomplished without a mechanical water pump or other ancillary water moving means, by allowing reactant gas bubbles, which leak through porous plates into the water stream, to escape through a vent in communication with at least one water outlet manifold at the top of the stack. Therein, water movement may also be in response to convection since the water within the water channel has a lower effective density due to the presence of gas bubbles therein and due to being warmer than the water outside of the stack. However, that device still requires plumbing external to the fuel cell stack to circulate the water from a water exit manifold back to a water inlet manifold.
Some of the foregoing difficulties are overcome by using an antifreeze solution for controlling the temperature of the fuel cell stack, particularly removing heat during normal operation. However, this results in the use of cooler plates every 2-4 fuel cells, which prevents having the fuel cells separated by fine pore plates, such as the water transport plates 23, 31, 34, 37. While the fuel cell described with respect to FIG. 1 allows water generated at the cathode of one cell to flow across water transport plates to the anode of the adjacent cell, and is quite effective in preventing dehydration of the PEM, this mechanism cannot occur in cells which are adjacent to solid coolers or separated by solid separator plates (used to separate those cells which do not have a cooler there between).
Some so-called passive water management concepts, which have no circulating liquid water, do not properly humidify the fuel at the inlet of the cell stack, which results in locally drying out the water transport plate and the proton exchange membrane. Drying out of the water transport plate may lead to gas leakage from the anode flow fields into the anode water flow fields. Drying out of the PEM results in degradation of the PEM due to attack by peroxide formed within the cell. If the fuel is humidified close to 100% relative humidity, these dry out problems are prevented; but this requires greater system complexity, volume and weight, as well as additional parasitic power loss.